This research is designed to investigate whether high carbohydrate diets are less fattening than high fat diets protein and caloric intakes are held constant. There is recent evidence to support the hypothesis that the increased prevalence of obesity in affluent countries is partially the result of the habitual intake of high fat diets. Our hypothesis is that high fat diets are more fattening than calorically equivalent high carbohydrate diets. Hormonal and metabolic factors have been discovered which can be predicted to contribute to this phenomenon. Therefore, high fat diets will contribute to obesity not only because of their greater palatability and intake, but even when fed in equal caloric amounts to high carbohydrate diets. Information on this point is particularly relevant and timely in view of dietary pronouncements by several government and private agencies, including the Senate Select Committee on Nutrition and Human Needs (1), the combined recommendations by the Department of Health, Education, and Welfare, and the Department of Agriculture (2) and the American Diabetes and Heart Associations. A long term caloric balance study is impossible in humans. It is, therefore, the specific aim of this proposal to study the impact on obesity in rats of altering the ratio of carbohydrate and fat in the diet while maintaining caloric and protein intakes the same. Besides this major aim, an attempt will be made to determine the roles played in this phenomenon by diet-induced alterations in energy expenditure, several hormones (thyroid hormones, insulin, growth hormone, and somatomedin-C), and sympapthetic nervous system activity.